FIG. 3 is a partial diagram of a nuclear power plant incorporating the present invention. In such a power plant, fluid is continuously transferred through a closed circulation loop between a nuclear reactor vessel 40 and a steam generator 45.
During power production, the pressurized fluid absorbs heat released by the thermonuclear reaction occurring in the reactor vessel 40. The heated fluid then flows through pipe 12 which is known appropriately as the hot leg of the fluid circulation loop. The hot leg pipe 12 delivers the hot fluid to the steam generator 45.
In the steam generator 45, the fluid circulates through a heat exchanger (not shown). The heat exchanger cools the fluid and uses the heat removed from the fluid to produce steam. This steam is eventually used to drive turbines and generate electricity.
After the circulating fluid is cooled by the heat exchanger, a circulation pump (not shown) removes the fluid from the steam generator 45 through line 44 and returns it to the reactor vessel 40 via inlet 46. The fluid is then reheated in the reactor vessel 40 and the cycle repeats.
This circulation of fluid is critical for the operation of the power plant in several respects. Not only does the circulating fluid deliver heat energy to the steam generator where the energy can be used to generate electricity, but the circulating fluid also acts as a coolant that prevents the reactor core in the reactor vessel 40 from over-heating.
Nuclear power plant systems, especially steam generators, require periodic maintenance. In particular, the fluid circulation system must be inspected for potential failures and nozzle dams must be installed and removed from the steam generator to allow inspection and mantenance to be performed in a dry environment.
In order to install and remove the nozzle dams, the fluid must be drained from the steam generator 45. This requires lowering the fluid level in the circulation loop, and consequently in the hot leg pipe. During such a maintenance period, the fluid continues to be heated by the reactor vessel 40, but is cooled by an alternate heat exchanger (not shown).
For safety reasons, it is important to know fairly accurately the level of fluid in the circulation loop during such a maintenance period. If the fluid level is not sufficiently lowered, workers performing the nozzle dam installation and removal in the steam generator 42 may be flooded. On the other hand, if the fluid level is reduced too much, the fluid will be unable to cool the reactor vessel 40 adequately.
Because the hot leg pipe is substantially in a horizontal plane and is directly connected to the reactor vessel 40, it is an ideal part of the circulation loop in which to monitor fluid level during system maintenance. Previous methods of monitoring the fluid level in nuclear power plants have proven inadequate. Such methods have either failed to provide a sufficiently accurate measurement of the fluid level or do not allow the fluid level to be monitored from a remote location, such as the power plant control room 48, where other safety equipment and parameters of the plant operation need to be monitored during maintenance periods.